1. Field of the Invention
This invention relates to heating cables, in particular self-regulating electrical heating cables.
2. Introduction to the Invention
Elongate electrical heating cables are well known for use in the freeze protection and temperature maintenance of pipes, tanks, and other substrates. Particularly useful elongate heating cables comprise (a) first and second elongate electrodes, (b) a plurality of resistive heating elements connected in parallel between said electrodes, and (c) an insulating jacket which surrounds the electrodes and heating elements. In addition, the heating cable often also comprises a metallic grounding layer, in the form of a braid or a tape, surrounding the insulating jacket, which serves to electrically ground the heating cable and provides mechanical protection. Because of the parallel construction of the heating elements, such heating cables may be cut to the appropriate length for use in each application.
For many applications it is preferred that the resistive heating elements comprise a conductive polymer, i.e. a polymer matrix in which is dispersed a particulate conductive filler. The conductive polymer preferably exhibits positive temperature coefficient (PTC) behavior, allowing the heating cable to be self-regulating. Two types of elongate heating cables are common. In the first, the conductive polymer is in the form of a continuous strip in which the electrodes are embedded. Such heaters are described, for example, in U.S. Pat. Nos. 3,858,144 (Bedard et al), 3,861,029 (Smith-Johannsen et al), 4,017,715 (Whitney et al), 4,242,573 (Batliwalla), 4,334,148 (Kampe), 4,334,351 (Sopory), 4,426,339 (Kamath et al), 4,574,188 (Midgley et al), and 5,111,032 (Batliwalla et al), and International Patent Publication No. WO91/17642 (Raychem Corporation, published Nov. 14, 1991). In the second type of heating cable, the conductive polymer is in the form of a continuous strip which is wrapped around the elongate electrodes, contacting the exposed electrodes alternately as it progresses down the length of the heating cable. In this configuration the electrodes are generally separated from one another by an insulating spacer. Alternatively, the electrodes can be wrapped around a core comprising the conductive polymer strip. Cables of this type are described in U.S. Pat. No. 4,459,473 (Kamath). The disclosure of each of these patents and publications is incorporated herein by reference.
Under normal operating conditions, an elongate heating cable is subject to physical stress and deformation as it is positioned in contact with the substrate to be heated. For example, when the substrate is a pipe or conduit, the cable is often wrapped around the pipe in a spiral manner. Furthermore, the cable must be wrapped around valves, joints, and other regions which must be heated. Although heating cables are flexible, certain conditions such as small pipe or valve diameters will require the cable to be bent or twisted. In addition, under normal use conditions, the heating cables undergo thermal cycling from relatively low temperatures to high temperatures. Both the physical stresses due to positioning on the substrate and the thermal stresses induced by the thermal cycling may cause changes in the heating cable. In particular, if the electrodes are positioned at opposite edges of a spacer, rather than embedded within a conductive polymer strip, the electrodes may move away from their position in the spacer, and/or may decrease contact with the conductive polymer strip wrapped around them.